Technical Reports SeriEs No.
Corrosion of Research Reactor Aluminium Clad Spent Fuel in Water
4I8
REACTOR ALUMINIUM CLAD
SPENT FUEL IN WATER
ALBANIA ALGERIA ANGOLA ARGENTINA ARMENIA AUSTRALIA AUSTRIA AZERBAIJAN BANGLADESH BELARUS BELGIUM BENIN BOLIVIA BOSNIA AND
HERZEGOVINA BOTSWANA BRAZIL BULGARIA BURKINA FASO CAMEROON CANADA
CENTRAL AFRICAN REPUBLIC CHILE CHINA COLOMBIA COSTA RICA CÔTE D’IVOIRE CROATIA CUBA CYPRUS
CZECH REPUBLIC DEMOCRATIC REPUBLIC
OF THE CONGO DENMARK
DOMINICAN REPUBLIC ECUADOR
EGYPT EL SALVADOR ERITREA ESTONIA ETHIOPIA FINLAND FRANCE GABON GEORGIA GERMANY GHANA
GUATEMALA HAITI HOLY SEE HONDURAS HUNGARY ICELAND INDIA INDONESIA
IRAN, ISLAMIC REPUBLIC OF IRAQ
IRELAND ISRAEL ITALY JAMAICA JAPAN JORDAN KAZAKHSTAN KENYA
KOREA, REPUBLIC OF KUWAIT
KYRGYZSTAN LATVIA LEBANON LIBERIA
LIBYAN ARAB JAMAHIRIYA LIECHTENSTEIN
LITHUANIA LUXEMBOURG MADAGASCAR MALAYSIA MALI MALTA
MARSHALL ISLANDS MAURITIUS
MEXICO MONACO MONGOLIA MOROCCO MYANMAR NAMIBIA NETHERLANDS NEW ZEALAND NICARAGUA NIGER NIGERIA NORWAY PAKISTAN PANAMA
PERU PHILIPPINES POLAND PORTUGAL QATAR
REPUBLIC OF MOLDOVA ROMANIA
RUSSIAN FEDERATION SAUDI ARABIA SENEGAL
SERBIA AND MONTENEGRO SEYCHELLES
SIERRA LEONE SINGAPORE SLOVAKIA SLOVENIA SOUTH AFRICA SPAIN
SRI LANKA SUDAN SWEDEN SWITZERLAND
SYRIAN ARAB REPUBLIC TAJIKISTAN
THAILAND
THE FORMER YUGOSLAV REPUBLIC OF MACEDONIA TUNISIA
TURKEY UGANDA UKRAINE
UNITED ARAB EMIRATES UNITED KINGDOM OF
GREAT BRITAIN AND NORTHERN IRELAND UNITED REPUBLIC
OF TANZANIA
UNITED STATES OF AMERICA URUGUAY
UZBEKISTAN VENEZUELA VIETNAM YEMEN ZAMBIA ZIMBABWE
The Agency’s Statute was approved on 23 October 1956 by the Conference on the Statute of the IAEA held at United Nations Headquarters, New York; it entered into force on 29 July 1957.
The Headquarters of the Agency are situated in Vienna. Its principal objective is “to accelerate and enlarge the contribution of atomic energy to peace, health and prosperity throughout the world’’.
© IAEA, 2003
Permission to reproduce or translate the information contained in this publication may be obtained by writing to the International Atomic Energy Agency, Wagramer Strasse 5, P.O. Box 100, A-1400 Vienna, Austria.
Printed by the IAEA in Austria December 2003 STI/DOC/010/418
CORROSION OF RESEARCH REACTOR ALUMINIUM CLAD
SPENT FUEL IN WATER
INTERNATIONAL ATOMIC ENERGY AGENCY VIENNA, 2003
Corrosion of research reactor aluminium clad spent fuel in water. — Vienna : International Atomic Energy Agency, 2003.
p. ; 24 cm. — (Technical reports series, ISSN 0074–1914 ; no. 418) STI/DOC/010/418
ISBN 92–0–113703–6
Includes bibliographical references.
1. Aluminum — Corrosion. 2. Nuclear fuel claddings. 3. Spent reactor fuels. I. International Atomic Energy Agency. II. Technical reports series (International Atomic Energy Agency) ; 418.
IAEAL 03-00341
This report documents the work performed in the IAEA Co-ordinated Research Project (CRP) on Corrosion of Research Reactor Aluminium Clad Spent Fuel in Water. The project consisted of the exposure of standard racks of corrosion coupons in the spent fuel pools of the participating research reactor laboratories and the evaluation of the coupons after predetermined exposure times, along with periodic monitoring of the storage water. The project was overseen by a supervisory group consisting of experts in the field, who also contributed a state of the art review that is included in this report.
The study was carried out in six laboratories in industrialized Member States and four laboratories in developing countries. Besides the basic goal of obtaining insight into the mechanisms of localized corrosion, a secondary goal was the transfer of know-how at the laboratory level from some of the more advanced laboratories and the supervisory group to the four institutes in devel- oping Member States. Localized corrosion mechanisms are notoriously difficult to understand, and it was clear from the outset that obtaining consistency in the results and their interpretation from laboratory to laboratory would depend on the development of an excellent set of experimental protocols.
The basic scope of the programme was originally formulated by the IAEA with the help of the supervisory group in early 1996. The design of the standard corrosion racks and corrosion coupons was based on a corrosion surveillance and monitoring programme for aluminium clad production reactor fuel that had already been established at the United States Department of Energy Savannah River Site (SRS) in Aiken, South Carolina. The CRP began formally with the signing of contracts and agreements in early 1996. The first Research Co-ordination Meeting (RCM) was held in August 1996. At this meeting the participants were briefed, the experimental protocols were developed and the first corrosion racks were distributed. Further RCMs were hosted by two of the participating laboratories in 1998 and 2000. Supervisory group meetings were also held at regular intervals to review the results obtained. The programme was completed and documented in mid-2001.
This report describes all of the work undertaken as part of the CRP and includes: a review of the state of the art understanding of corrosion of research reactor aluminium alloy cladding materials; a description of the standard corrosion racks, experimental protocols, test procedures and water quality monitoring; the specific contributions by each of nine participating labora- tories; a compilation of all experimental results obtained; and the supervisory group’s analysis and discussion of the results, along with conclusions and recommendations.
(SRS, USA), A.B. Johnson, Jr. (Pacific Northwest National Laboratory, Hanford, Washington, USA), L.V. Ramanathan (Instituto de Pesquisas Energéticas e Nucleares, São Paulo, Brazil) and I. Vidovszky (KFKI Atomic Energy Research Institute, Budapest, Hungary), who were the major contrib- utors to the drafting and review of Chapters 1 to 4 of this publication and who together with the Scientific Secretary of the CRP formed the supervisory group. The IAEA officer responsible for the compilation of this report was I.G. Ritchie of the Division of Nuclear Fuel Cycle and Waste Technology.
EDITORIAL NOTE
Although great care has been taken to maintain the accuracy of information contained in this publication, neither the IAEA nor its Member States assume any respon- sibility for consequences which may arise from its use.
The mention of names of specific companies or products (whether or not indicated as registered) does not imply any intention to infringe proprietary rights, nor should it be construed as an endorsement or recommendation on the part of the IAEA.
SUMMARY . . . 1
CHAPTER 1. BACKGROUND OF THE IAEA CO-ORDINATED RESEARCH PROJECT . . . 7
1.1. Introduction . . . 7
1.2. Storage of research and test reactor spent fuel worldwide . . . 8
1.3. IAEA Co-ordinated Research Project . . . 10
1.3.1. Details of the corrosion monitoring programme . . . 11
1.3.2. Initiation of the Co-ordinated Research Project . . . 12
1.3.3. Monitoring corrosion racks at research reactor storage basins . . . 12
1.3.4. Results . . . 13
1.3.4.1. Comisión Nacional de Energía Atómica, Centro Atómico Constituyentes (CNEA-CAC), Buenos Aires, Argentina . . . 13
1.3.4.2. Instituto de Pesquisas Energéticas e Nucleares (IPEN), São Paulo, Brazil . . . 14
1.3.4.3. China Institute of Atomic Energy, Beijing, China . . . 15
1.3.4.4. KFKI Atomic Energy Research Institute, Budapest, Hungary . . . 15
1.3.4.5. Bhabha Atomic Research Centre, Trombay, India . . . 16
1.3.4.6. Pakistan Institute of Nuclear Science and Technology, Islamabad, Pakistan . . . 17
1.3.4.7. Research Institute of Atomic Reactors, Dimitrovgrad, Russian Federation . . . 17
1.3.4.8. Russian Research Center, Kurchatov Institute, Moscow, Russian Federation . . . 18
1.3.4.9. Office of Atomic Energy for Peace, Bangkok, Thailand . . . 18
1.3.5. General comments on the CRP . . . 19
1.4. SRS corrosion surveillance programme . . . 19
1.4.1. Background . . . 20
1.4.2. Component immersion tests . . . 21
1.4.3. Research and test reactor spent fuel corrosion surveillance . . . 23
1.4.3.1. Corrosion racks and test coupons . . . 23
1.4.3.2. Schedule for withdrawal and analysis . . . 26
1.4.3.3. Results . . . 27
1.5. Conclusions . . . 31
References to Chapter 1 . . . 33
CHAPTER 2. STATE OF THE ART REVIEW ON ALUMINIUM CORROSION . . . 35
2.1. Introduction . . . 35
2.2. Fundamental factors affecting corrosion . . . 36
2.2.1. Oxide films on aluminium . . . 36
2.2.2. Kinetics . . . 37
2.2.3. Types of corrosion . . . 37
2.3. Environmental factors affecting aluminium corrosion . . . 40
2.3.1. Influence of water composition . . . 41
2.3.2. Conductivity of water . . . 41
2.3.3. Effect of pH . . . 43
2.3.4. Effect of impurities . . . 43
2.3.5. Copper . . . 44
2.3.6. Bicarbonate . . . 45
2.3.7. Sulphates . . . 45
2.3.8. Oxygen . . . 46
2.3.9. Temperature . . . 46
2.4. Pitting rate index . . . 47
2.5. Conclusions . . . 48
References to Chapter 2 . . . 49
CHAPTER 3. GUIDELINES FOR CORROSION PROTECTION OF RESEARCH REACTOR ALUMINIUM CLAD SPENT NUCLEAR FUEL IN INTERIM WET STORAGE . . . 51
3.1. Introduction . . . 51
3.2. Scope . . . 52
3.3. Corrosion experience with aluminium clad spent fuel in wet storage . . . 52
3.4. Types of corrosion encountered in spent fuel storage basins . . . 53
3.4.3. Crevice corrosion . . . 54
3.4.4. Pitting corrosion . . . 54
3.4.5. Hydrogen blisters . . . 55
3.5. Proposed guidelines for corrosion protection of aluminium clad spent fuel in wet storage . . . 56
3.5.1. Water chemistry . . . 56
3.5.2. Operational practices . . . 58
References to Chapter 3 . . . 61
CHAPTER 4. CRP TEST MATERIALS, RACKS AND EXPERIMENTAL PROTOCOLS . . . 63
4.1. Introduction . . . 63
4.2. Materials, coupons and racks . . . 64
4.2.1. Batch I racks . . . 65
4.2.2. Batch II racks . . . 70
4.3. Test protocol . . . 71
4.3.1. Preassembly . . . 71
4.3.2. Assembly . . . 72
4.3.3. Immersion in the storage basin . . . 73
4.3.4. Exposure interval . . . 73
4.3.5. Removal and examination of coupons . . . 73
4.3.6. Post-storage detailed examination . . . 74
4.3.7. Final report . . . 74
4.3.7.1. Preparation . . . 74
4.3.7.2. Evaluation . . . 75
4.4. Concluding remarks . . . 75
CHAPTER 5. CORROSION OF RESEARCH REACTOR ALUMINIUM CLAD SPENT FUEL IN WATER AT VARIOUS SITES IN ARGENTINA . . . 77
5.1. General introduction . . . 77
5.2. First stage: Rack 1 . . . 78
5.2.1. Introduction . . . 78
5.2.2. Experimental set-up . . . 78
5.2.3.2. Metallographic examination . . . 83
5.2.3.3. Water chemistry . . . 90
5.2.4. Discussion . . . 93
5.3. Conclusions of the first stage . . . 94
5.4. Extended programme . . . 94
5.4.1. Introduction . . . 94
5.4.2. Experimental set-up . . . 95
5.4.3. Results . . . 100
5.4.3.1. Water chemistry . . . 100
5.4.3.2. Appearance of the samples . . . 100
5.4.3.3. Metallography . . . 108
5.4.4. Discussion . . . 109
5.5. Conclusions . . . 113
References to Chapter 5 . . . 116
CHAPTER 6. CORROSION BEHAVIOUR OF ALUMINIUM ALLOYS IN THE SPENT FUEL STORAGE SECTION OF THE IEA-R1 RESEARCH REACTOR, IPEN, SÃO PAULO, BRAZIL . . . 117
6.1. Introduction . . . 117
6.1.1. The IEA-R1 research reactor . . . 117
6.1.2. Spent fuel storage . . . 118
6.1.3. Fuel assessment — visual inspection of spent fuel assemblies . . . 119
6.1.4. Corrosion experience related to IEA-R1 reactor fuel and aluminium alloys . . . 121
6.2. The IAEA CRP . . . 122
6.2.1. IAEA rack 1 . . . 122
6.2.2. Results of the first inspection of rack 1 . . . 123
6.2.3. IAEA racks 2A, 2B, 3A and 3B . . . 124
6.2.4. The IPEN rack . . . 126
6.3. Results . . . 128
6.4. Recommendations by the CRP participants from IPEN . . . 128
References to Chapter 6 . . . 129
BASIN OF THE CHINA INSTITUTE OF ATOMIC
ENERGY, BEIJING, CHINA . . . 131
7.1. Introduction . . . 131
7.2. Experiment . . . 131
7.2.1. Test coupons and racks . . . 131
7.2.2. Spent fuel basin . . . 132
7.2.3. Reactor and spent fuel . . . 134
7.2.4. Basin water monitoring . . . 134
7.3. Experimental details . . . 135
7.3.1. Water chemistry parameters, radioactivity and radiation level . . . 135
7.3.2. Visual observation and inspection with a magnifying glass . . . 136
7.3.3. Photographic record . . . 137
7.3.4. Metallographic analyses . . . 139
7.4. Conclusions . . . 140
References to Chapter 7 . . . 141
CHAPTER 8. CORROSION OF ALUMINIUM ALLOY TEST COUPONS IN THE SPENT FUEL BASIN OF THE BUDAPEST RESEARCH REACTOR AT AEKI, BUDAPEST, HUNGARY . . . 143
8.1. Introduction . . . 143
8.2. Reactor and spent fuel storage pool . . . 143
8.3. Investigations . . . 146
8.3.1. Inspection and evaluation of rack 1 (after 6 and 12 months) . . . 146
8.3.2. Inspection and evaluation of racks 2 and 3 (after 12 and 24 months) . . . 146
8.3.3. Preparation of the second set of racks . . . 147
8.4. Results . . . 148
8.4.1. Rack 2 . . . 148
8.4.2. Rack 3 . . . 149
8.5. Conclusions . . . 151
STORAGE POOL AT BARC, MUMBAI, INDIA . . . 153
9.1. Introduction . . . 153
9.2. Experimental procedure . . . 154
9.2.1. Coupons received at the Budapest RCM . . . 154
9.2.2. Coupons received at the São Paulo RCM . . . 155
9.3. Observations . . . 156
9.3.1. Coupons received at the Budapest RCM . . . 156
9.3.2. Coupons received at the São Paulo RCM . . . 157
9.4. Discussion . . . 159
9.5. Conclusions . . . 160
Acknowledgements . . . 161
Reference to Chapter 9 . . . 161
CHAPTER 10. CORROSION OF ALUMINIUM COUPONS IN THE FUEL STORAGE BAY OF PINSTECH, ISLAMABAD, PAKISTAN . . . 163
10.1. Introduction . . . 163
10.2. Description of procedures . . . 163
10.3. Experimental procedure . . . 165
10.3.1. Preparation of rack assembly . . . 165
10.3.2. Immersion of rack 2 in the pool . . . 166
10.3.3. Basin water chemistry . . . 166
10.3.4. Radiation measurements . . . 166
10.4. Results and discussion . . . 166
10.4.1. Monthly inspections . . . 166
10.4.2. Basin water chemistry . . . 167
10.4.3. Radiation measurements . . . 168
10.4.4. Removal of rack 2 . . . 168
10.4.4.1. Observations at the site . . . 168
10.4.4.2. Detailed examination in the laboratory . . . 168
10.4.4.3. Post-exposure detailed examination . . . 169
10.4.5. Permanent withdrawal of rack 3 . . . 170
10.5. Conclusions . . . 170
References to Chapter 10 . . . 177
SPENT FUEL POOL OF THE MIR REACTOR,
DIMITROVGRAD, RUSSIAN FEDERATION . . . 179
11.1. Introduction . . . 179
11.2. Coupon preparation . . . 179
11.3. Main features of spent fuel pool operation in the MIR reactor . . . . 180
11.4. Results . . . 181
11.4.1. Investigation of coupon surfaces . . . 183
11.5. Conclusions . . . 188
CHAPTER 12. CORROSION OF ALUMINIUM ALLOY COUPONS IN THE IR-8 REACTOR SPENT FUEL STORAGE BASIN AT KURCHATOV INSTITUTE, MOSCOW, RUSSIAN FEDERATION . . . 189
12.1. Introduction . . . 189
12.2. Description of the aluminium alloy coupons of the three racks . . . . 190
12.2.1. Rack 1 . . . 190
12.2.2. Racks 2 and 3 . . . 190
12.3. Reactor operating conditions . . . 190
12.4. Results and discussion . . . 192
12.5. Conclusions . . . 195
Acknowledgements . . . 196
Reference to Chapter 12 . . . 196
CHAPTER 13. CORROSION OF ALUMINIUM ALLOY COUPONS IN THE SPENT FUEL BASIN AT THE OFFICE OF ATOMIC ENERGY FOR PEACE, BANGKOK, THAILAND . . . 197
13.1. Introduction . . . 197
13.2. Experiment . . . 197
13.3. Procedure . . . 198
13.3.1. Water basin chemistry . . . 198
13.3.2. Radiation field . . . 198
13.3.3. Coupon preparation . . . 198
13.4.1. Basin water chemistry . . . 199
13.4.2. Coupon monitoring . . . 199
13.4.3. Pit measurements . . . 200
13.4.4. Glass ampoule coupons . . . 207
13.5. Conclusions . . . 207
References to Chapter 13 . . . 207
PARTICIPANTS IN THE CRP . . . 209
Aluminium clad spent nuclear fuel from research and test reactors worldwide is currently being stored in water filled basins while awaiting final disposition. Much of this fuel was provided to the various countries by the United States of America as part of the Atoms for Peace programme in the early 1950s. Other fuel was provided by the former Soviet Union. The spent fuel has been in water at the reactor sites for up to 40 years, in some cases, awaiting shipment back to the USA or to the Russian Federation.
As a result of corrosion issues that developed from the long term storage of the aluminium clad fuel, the IAEA implemented in 1996 a Co-ordinated Research Project (CRP) on the Corrosion of Research Reactor Aluminium Clad Spent Fuel in Water. During the initial meeting of experts to develop the CRP, it was discovered that a comprehensive programme on the corrosion of aluminium clad nuclear fuel was already under way at the US Department of Energy (USDOE) Savannah River Site (SRS) in Aiken, South Carolina. This programme did not involve research reactor fuel per se but was set up to address the corrosion of aluminium clad production reactor fuel, which had become caught in the nuclear pipeline when the USA decided to terminate reprocessing of the fuel in question. This programme, begun in the early 1990s to clean up the SRS spent fuel basins and to implement a corrosion monitoring and surveillance programme, was already well established at SRS. It was clear that the CRP would benefit tremendously from the experience of the SRS programme. The SRS joined the CRP, and its chief scientific investigator became a key member of the CRP supervisory group. From the beginning the CRP was designed to complement and enhance the SRS programme and to transfer knowledge gained from studies of the corrosion of production reactor fuel to research reactor fuel and vice versa.
The scientific investigations undertaken during the CRP involved ten institutes in nine countries. The IAEA furnished corrosion surveillance racks with aluminium alloys generally used in the manufacture of nuclear fuel cladding. The individual countries supplemented these racks with additional racks and coupons specific to materials in their storage basins.
The initial corrosion racks provided by the IAEA were immersed in late 1996 in water storage pools with a wide range of water chemistry and environ- mental conditions, and were monitored for corrosion over a period of time. The results of these early observations were reported after 18 months at the second research co-ordination meeting (RCM) of the CRP, held in São Paulo, Brazil.
Pitting and crevice corrosion were the primary corrosion mechanisms observed. Corrosion by deposition of iron and other cathodic particles on the
surface of the aluminium fuel was observed in a number of basins where these particles were seen floating in the water. All corrosion mechanisms were galvanically accelerated in stainless steel–aluminium coupled coupons.
Corrosion was not generally observed in those basins whose water conductivity was near 1 µS/cm and whose chloride ion concentration was in the ppb range.
Pitting caused by particle deposition was seen in one case, even though the water was of the highest quality.
Additional corrosion racks were provided to the CRP participants in March 1998 at the second RCM. Most of these racks had been immersed in the individual basins by mid-1998. The surveillance racks were monitored visually for corrosion, and when corrosion was detected, the coupons were removed from the water and analysed. As found in earlier testing, water quality proved to be the key to good performance. Crevice corrosion was seen between most of the crevice couples as expected, because the pH was lower by 0.5–1.0 unit in the crevice. In poorer quality water, further corrosion was observed, especially between bimetallic crevice coupons, to the extent that coupons had to be forced apart. The results of the individual participating laboratories were presented at the third and final RCM, held in Bangkok, Thailand, in October 2000.
As already mentioned, corrosion of aluminium clad spent fuel has been studied extensively in the USA at SRS. Corrosion surveillance racks containing a large number of aluminium alloys have been immersed in four different water storage basins under a wide variety of conditions and for long times of exposure. Results similar to those obtained in the CRP were observed and are also presented in this report. Significant pitting and galvanic corrosion were observed in the early 1990s, when water quality was poor. Improved basin management procedures were undertaken and the water quality was quickly improved. Under the improved conditions, no pitting corrosion has been seen in any of the fuel storage basins at SRS since 1994.
OUTLINE OF THE REPORT
The detailed background and designs of the CRP and SRS programmes are presented in Chapter 1.
A thorough state of the art literature review on the corrosion of aluminium alloys was compiled by the IAEA in 1998. This review was published in IAEA-TECDOC-1012, Durability of Spent Nuclear Fuels and Facility Components in Wet Storage. It covered a wide range of quantitative and semi-quantitative data on cladding alloys used in nuclear fuel elements and assemblies, and included separate sections on corrosion of aluminium, zirconium, stainless steel, carbon steels and copper alloys in a wet storage
environment. Relevant sections of this document that apply to the aluminium alloys and fuels predominantly used in fuel for research and test reactors have been updated and are presented in Chapter 2. This chapter contains a discus- sion of the fundamentals of aluminium alloy corrosion in the wet storage of spent nuclear fuel throughout the world, examines the effects of variables in the storage environment and presents the results of corrosion surveillance testing activities at SRS, as well as discussions of fuel storage basins at other production sites of the USDOE.
On the basis of the knowledge gained during the CRP and the corrosion surveillance programme at SRS, a fundamental understanding of the corrosion of aluminium clad spent fuel has been developed. From this understanding, guidelines for the corrosion protection of aluminium cladding alloys have been developed. These guidelines are presented in Chapter 3.
Chapter 4 presents the details of the corrosion coupons, racks and exper- imental protocols developed for the CRP.
Chapters 5–13 present the individual reports of the participating insti- tutes, with the exception of SRS results, which have been incorporated into Chapters 1–3. Each report originally contained photographs of the corrosion racks and coupons as well as descriptions of the alloys and their preparation, and the as-received surface features of the coupons. Since this information is discussed in detail in Chapter 4, the participants’ reports have been revised to avoid repetition where possible, without removing important technical data. An initial attempt was made to investigate the corrosion weight gain/loss data of individual coupons. This required disassembly of the coupon racks for weighing and reassembling, which disrupted long term localized corrosion data. Since general corrosion has never been a serious problem in fuel storage basins, the early weight gain/loss data have not been included here.
GENERAL COMMENTS ON THE CRP
(a) The pH of the water and the specimens inside the glass ampoules provided to each participant did not show any changes. These specimens were designed to evaluate radiation effects.
(b) The colour of the exposed aluminium alloy surfaces varied from metallic bright to dark grey. The extent to which the surfaces darkened was dependent on the alloy composition.
(c) Sediments were observed on the top surfaces of many coupons.
(d) A number of participants reported corrosion along the outer rim of the coupons. This would be expected from end grain attack on cut surfaces.
(e) The highly polished coupons were more resistant to corrosion than the as-machined coupons.
(f) The crevice/bimetallic couples were often stuck together with corrosion products and required forcible separation.
(g) The pH in the crevice was generally 0.5–1.0 unit less than in the bulk water.
(h) Pits of <0.5 mm diameter were observed on the aluminium at regions in contact with the ceramic separator.
(i) Sediments on the top surfaces of aluminium alloy coupons caused pitting.
No pits were observed on the bottom surfaces of these coupons.
(j) Surface features of coupons exposed for 13 months were similar to those of coupons exposed for 25 months. This timescale had no significant effect on aluminium coupon corrosion.
(k) The crevices of the aluminium alloy couples were stained but not pitted, whereas the aluminium–stainless couples were heavily pitted.
CONCLUSIONS
A large database on corrosion of aluminium clad materials has been generated from the CRP and the SRS corrosion surveillance programme. An evaluation of these data indicates that the most important factors contributing to the corrosion of the aluminium are:
(1) High water conductivity (100–200 µS/cm);
(2) Aggressive impurity ion concentrations (Cl–);
(3) Deposition of cathodic particles on aluminium (Fe, etc.);
(4) Sludge (containing Fe, Cl–and other ions in concentrations greater than ten times the concentrations in the water);
(5) Galvanic couples between dissimilar metals (stainless steel–aluminium, aluminium–uranium, etc);
(6) Scratches and imperfections (in protective oxide coating on cladding);
(7) Poor water circulation.
These factors operating both independently and synergistically may cause corrosion of the aluminium. The single most important key to preventing corrosion is maintaining good water chemistry. Water conductivity near 1 µS/cm generally ensures that aggressive impurity ions such as chlorides are in the ppb range. When chemistry is maintained in this regime, corrosion of aluminium alloys is minimized.
Good water chemistry alone does not always guarantee that corrosion will be prevented, as shown by the extensive testing conducted in the Argentine
storage pools, where iron oxide particles deposited from the water caused pitting even in high purity water. This has also been seen in other fuel storage basins. Corrosion mechanisms involved in this pitting can be both galvanic and oxygen depletion cells.
The CRP has succeeded in making all the participating countries more aware that the successful wet storage of aluminium clad spent fuel does not come about automatically but requires diligence in maintaining high quality water conditions. Moreover, from papers written and published during the CRP by the participants and from the presentation of some of the results at interna- tional conferences, the whole research reactor community is now more fully aware of the susceptibility of aluminium cladding to localized corrosion and the measures that can be taken to minimize it.
RECOMMENDATIONS
Any continuation of the research initiated during this CRP should concentrate on fuel storage basins that have demonstrated significant corrosion problems and will therefore provide additional and much needed insight into the fundamentals of localized corrosion. A better understanding of the funda- mental mechanisms will allow the prediction of corrosion rates under different combinations of environmental parameters, enabling storage pool operators to better control those parameters essential to the safe and efficient interim storage of aluminium clad spent fuel.
More comprehensive research is recommended in the following areas:
(a) Evaluation of the effect of dust sediments on the corrosion of coupons and its implication for the corrosion of fuel cladding;
(b) Identification of the different aluminium alloys and other metals presently in use in spent fuel basins and experiments designed to evaluate the effect of specific bimetallic couples;
(c) Evaluation of the effect of hydrodynamic conditions on coupon and fuel cladding corrosion;
(d) Evaluation of the effects of water quality parameters on localized corrosion of aluminium fuel cladding in the wide range that exists between known poor water chemistry conditions and optimum conditions.
BACKGROUND OF THE IAEA CO-ORDINATED RESEARCH PROJECT
1.1. INTRODUCTION
Test and research reactor fuel is currently being shipped from within the United States of America and from locations all over the world for interim storage in water filled basins at the Savannah River Site (SRS) in Aiken, South Carolina, USA. The fuel was provided by the USA to many of the countries as a part of the Atoms for Peace programme in the early 1950s. Now, as part of the non-proliferation policy on foreign research reactor spent nuclear fuel of the US Department of Energy (USDOE), much of this fuel is being sent back from research and test reactors in Europe, Asia and Latin America. This fuel has been in water storage at the reactor sites for times ranging from a few years to over 40 years. Most of the fuel assemblies were manufactured from U–Al alloy and clad with aluminium. The quality of water in the fuel storage basins ranges from highly deionized water to untreated and uncirculated water. In the latter extremely aggressive environments, the aluminium clad fuel is very susceptible to pitting corrosion. In the early 1990s, corrosion of aluminium clad fuel was an issue at several of the storage basins in the USA, and has also been seen on materials test reactor (MTR) type research reactor fuel scheduled for shipment back to SRS [1.1].
With aluminium clad fuel corrosion issues starting to appear in wet spent fuel storage basins around the world, the IAEA formulated a corrosion surveil- lance programme in late 1994. This scientific investigation was implemented in 1996 as part of an IAEA Co-ordinated Research Project (CRP) on Corrosion of Research Reactor Aluminium Clad Spent Fuel in Water. Scientists from countries worldwide were invited to participate [1.2]. The results of the CRP were presented at a final research co-ordination meeting (RCM) in Bangkok, Thailand, in October 2000 and are documented in Chapters 5–13.
This report is a summary and overview of the scientific investigations of this CRP as carried out in the nine participating countries. The results of corrosion surveillance activities in the individual fuel storage basins of these countries are discussed in detail. On the basis of the knowledge gained from the overall results of this project, a set of Guidelines for Corrosion Protection of Research Reactor Aluminium Clad Spent Nuclear Fuel in Interim Wet Storage were developed and are presented in Chapter 3.
1.2. STORAGE OF RESEARCH AND TEST REACTOR SPENT FUEL WORLDWIDE
According to the IAEA database on Nuclear Research Reactors in the World, as of October 2003 there were 272 research reactors in operation, with 214 reactors shut down, 168 decommissioned, 9 under construction and 8 in the planning stages [1.3]. It is instructive to see how these are divided between the developed or industrialized countries of the world and the developing countries. In the industrialized countries, there are 193 research reactors in operation, with 230 reactors shut down, 106 decommissioned, 4 under construc- tion and 3 in the planning stages, while in the developing countries, there are 85 in operation, with 28 shut down, 12 decommissioned, 5 under construction and 5 in the planning stages. The age distribution of operating research reactors peaks at between 35 and 40 years, with 61% of them more than 30 years old.
The most common form of spent fuel storage for these research reactors is at- reactor pools or basins. Some of the reactors have auxiliary away-from-reactor pools or dry wells. At some of these auxiliary facilities, the trend has been to shift some fuel from wet to dry storage to avoid the expense of water treatment facilities and maintenance.
Many of the spent nuclear fuel assemblies from Western research reactors are MTR box type, involute plate, tubular, rod cluster or pin assemblies. A typical MTR type fuel assembly is shown in Fig. 1.1. Russian designed research reactors utilize fuel assemblies of different geometrical types, which can be divided into two main groups — multitube assemblies and multirod assemblies.
Most of the fuel core is manufactured from U–Al alloy initially enriched to
≥20% (HEU) or <20% (LEU). The cladding alloys of Western fuel types are usually 6061 or 1100 grade aluminium1.1 ranging in thickness from 0.375 to 0.75 mm. The irradiated aluminium clad assemblies are generally stored in light water filled basins where the corrosion resistance of the aluminium is usually good as long as the water chemistry is maintained at high purity levels. If corrosion is a problem, it is usually in the form of pitting. The most important water parameters affecting the corrosion of these alloys are normally conduc- tivity and aggressive impurity (e.g. chloride ion) concentrations. A tightly adhered aluminium oxide (boehmite) coating formed on the fuel plates during irradiation provides extra corrosion protection in basin storage as long as it remains intact.
As part of the USDOE’s programme to bring research reactor fuel back to the USA, engineers from SRS inspected the spent fuel for corrosion and
1.1In this book, the numbers given for aluminium alloys refer to the Aluminum Association (AA) specifications.
mechanical damage [1.4]. Over 1700 aluminium clad assemblies were individu- ally examined using video and underwater cameras to record their condition at the fuel storage sites. A wide range of physical conditions of these spent nuclear fuel assemblies were observed. Many of the assemblies after 20 years of storage were in pristine condition. Other fuel assemblies had extensive nodular corrosion products clearly visible on the outer fuel plates, as seen in Fig. 1.2.
Removal of the nodules revealed extensive pitting corrosion that had breached the 0.375 mm aluminium cladding. Pitting corrosion that had pene- trated the aluminium cladding to the fuel meat was found on approximately 7% of the total number of assemblies inspected by SRS.
In addition to visual inspection of fuel stored in research reactor basins outside the USA, gamma spectroscopy was used to measure the radionuclide release from the U–Al fuel in water. Water samples were drawn from the
FIG. 1.1. Typical MTR type spent nuclear fuel element.
FIG. 1.2. Nodular corrosion products and exposed pits on MTR type spent nuclear fuel.
shipping cask, which contained 40 assemblies, before and after a 4 h minimum rest time. Measurements of the 137Cs activity in the cask water were used to determine a difference of about 10 pCi (0.37 Bq)/mL, corresponding to a release rate of 0.9 µCi (3.33 × 104Bq)/h into a 100 gallon (379 L) cask. Even though the fuel had known cladding penetrations, the release of radioactivity from this U–Al alloy fuel was found to be barely detectable and was far below the SRS site limit of 20.7 µCi (7.66 × 105Bq)/h per cask [1.5].
1.3. IAEA CO-ORDINATED RESEARCH PROJECT
In December 1994, a meeting of corrosion experts was held at IAEA Headquarters in Vienna as part of an ongoing CRP entitled Irradiation Enhanced Degradation of Materials in Spent Fuel Storage Facilities. During this meeting, spent fuel corrosion issues at SRS and other sites in the USA were discussed by the SRS participant with the IAEA and the European participants.
In the early 1990s, corrosion of aluminium clad spent nuclear fuel stored in light water filled basins became a major concern, and programmes were implemented at the sites to improve fuel storage conditions. The Savannah River Technology Center (SRTC), along with the Spent Fuel Storage Division at SRS, established a corrosion surveillance programme in support of shipment of the research reactor fuel. Details of this 20 year programme were presented.
As a result of these discussions and the recommendations of an advisory group, the IAEA established the CRP on Corrosion of Research Reactor Aluminium Clad Spent Fuel in Water. The CRP was designed to address several issues raised by vulnerability assessments conducted at some of the spent fuel storage sites. Its objectives were to:
(a) Establish uniform practices for corrosion monitoring and surveillance;
(b) Provide a technical basis for continued wet storage of research reactor spent fuel;
(c) Collect data to help in the prediction of lifetimes of fuel handling tools and storage racks;
(d) Establish a uniform basis for the characterization of water in fuel storage basins.
Nine countries — Argentina, Brazil, China, Hungary, India, Pakistan, the Russian Federation (two different sites), the USA and Thailand — were invited by the IAEA to participate in the CRP. Research agreements or contracts with institutes in these countries were put in place for work to be performed, and the IAEA provided a detailed work package and standard corrosion test coupons to each participant.
The CRP was based on a corrosion surveillance programme developed and planned for several USDOE spent fuel storage basins in the USA. The proposed US programme was an extensive national effort to monitor the corrosion of different aluminium alloys in racks at storage basins at SRS, the Idaho nuclear site, USDOE Hanford and the West Valley site. The US programme was essentially implemented at SRS, but the other sites opted for smaller individual programmes [1.6]. With a limited budget, the IAEA version of the corrosion monitoring programme was necessarily smaller and was scaled down with respect to the number of racks and coupons. The programme, however, designed to develop basic information on the corrosion of aluminium clad alloys in spent fuel storage environments and to increase awareness of the fact that water quality is the key to successful long term storage of spent nuclear fuel.
1.3.1. Details of the corrosion monitoring programme
The materials selected for testing were representative of typical aluminium cladding alloys used in research reactor fuel, handling tools and storage racks. Aluminium alloy types 5086, 1100, 6061, 6063 and SZAV-1 (throughout this book, aluminium alloys are referred to by their Aluminum Association (AA) numbers), and stainless steel type 316 were produced by the KFKI Atomic Energy Research Institute (AEKI) in Budapest, Hungary, for use by the participants. A single heat of each alloy was used to make the test coupons. In addition to the IAEA rack of corrosion coupons, many of the participants immersed an additional rack in their basins. This rack consisted of coupons of alloys specific to their research reactor fuel and handling tools.
Chapter 4 provides a more detailed discussion of the racks and testing protocol.
Test plans for the monitoring were specific to the individual sites and included details for the assembly, exposure, disassembly and evaluation of the coupons. As a minimum evaluation, each site was asked to weigh, clean and photograph each coupon before exposure, and each assembled coupon rack before and after exposure. Detailed metallurgical evaluation of any corrosion was suggested where possible.
Each participant was requested to measure the parameters of their basin water on at least a quarterly basis. The following parameters were to be measured, if possible: temperature, pH, conductivity, chlorides, nitrates, nitrites, sulphates and basin radioactivity. Investigating the correlation between corrosion of the coupons and water parameters was one of the overall goals of this project.
1.3.2. Initiation of the Co-ordinated Research Project
The first RCM of the CRP was held in Budapest on 7–9 August 1996 at AEKI. The nine countries invited to participate were carrying out storage of aluminium clad spent fuel in water filled basins. A presentation was made by each participant describing the current status of the research reactors in their countries and the condition of the spent fuel stored in their basins. The condition of the fuel ranged from being corrosion free in some cases to showing extensive nodular corrosion in others. The SRS corrosion surveil- lance programme, which served as a model for the IAEA project, was discussed in detail. In addition, a representative from PNNL made a presen- tation on fuel cladding and storage component corrosion experience at the Hanford site.
Details of the research to be conducted during the CRP were presented by the IAEA and each participant was given one corrosion rack to take back to their individual reactor and fuel storage sites. The first RCM of the CRP was successful in terms of the extensive technical exchange among the scientists from the participating countries. Plans were developed in Budapest to hold the second RCM after about 18 months of research. Communication by e-mail was selected as the means to address issues and answer questions among the scientists and the IAEA during the CRP.
1.3.3. Monitoring corrosion racks at research reactor storage basins
After the first RCM, the participants formulated individual test plans specific to their spent fuel storage basins. A general test protocol for conducting the programme was provided by the IAEA. This protocol included instructions for preassembly, assembly and immersion of the corrosion racks in the storage basin, exposure intervals, and removal and examination.
The individual participants were asked to prepare the corrosion racks and to begin the exposure as soon as possible. Most of the racks were assembled and immersed in fuel storage pools well before the end of 1996.
Because of the limited number of racks available, participants were asked to make periodic visual examinations of the coupons to determine whether active corrosion was obvious. If corrosion was visible, the racks were to be removed from the water and the coupons photographed. The racks were then returned to the basin for additional exposure. Water chemistry measurements were made on a periodic basis and monthly visual inspections were performed by most of the participants.
1.3.4. Results
The second RCM was held in São Paulo, Brazil, in March 1998, and the third and final RCM in Bangkok, Thailand, in October 2000. Each participant presented his/her test results at these meetings. A brief summary of the presen- tations is given below. The final reports of the individual countries are given in Chapters 5–13, with the exception of the SRS results, which have been incor- porated into Chapters 1, 2 and 3. Each report originally contained photographs of the corrosion racks and coupons as well as descriptions of the alloys, their preparation and the as-received surface features of the coupons. Since this information is discussed in detail in Chapter 4, the participants’ reports have been revised to avoid repetition where possible, without removing important technical data. An initial attempt was made to investigate corrosion weight gain/loss data of individual coupons. This required disassembly of the coupon racks for weighing and reassembling, which disrupted long term localized corrosion data. Since general corrosion has never been a serious problem in fuel storage basins, the early weight gain/loss data have not been included here.
1.3.4.1. Comisión Nacional de Energía Atómica, Centro Atómico Constituyentes (CNEA-CAC), Buenos Aires, Argentina
Rack 1 was immersed in one of the open channels at the Central Storage Facility (CSF), in Ezeiza, at about one metre from a spent fuel assembly and sharing the same water. The rack was inspected after 60 days of exposure and was found to be coated with a thin brownish layer along with some dark particles. Corrosion products were visible on a number of coupons. Several white nodules were observed that, when cleaned, revealed pits in the base metal. Pitting was associated with the particles. Crevice coupons were stuck together and were difficult to separate because of corrosion. Teflon spacers were used to centre the rack in the cylindrical channel. There was extensive crevice corrosion under these spacers. The SZAV-1 aluminium alloy showed a lesser tendency to corrode and the 6061 aluminium–stainless steel galvanic couples showed the highest. The water quality was aggressive for the storage of aluminium clad alloys, with a conductivity of 74 µS/cm and a chloride ion content of 14.8 ppm.
In mid-1998, six additional racks containing aluminium crevice and galvanic couple coupons were immersed at the CSF, in the RA3 decay pool, the RA6 reactor pool and the RA6 decay pool. These different pools provided a wide range of water chemistry conditions. Conductivity ranged from 227 µS/cm at the CSF down to 1.8 µS/cm in the RA6 reactor pool. The chloride ion content ranged from 16 ppm at the CSF to less than 0.5 ppm in the reactor pool. The
racks immersed in the basins included coupons provided by the IAEA and aluminium alloy coupons used in the manufacture of Argentine fuel. Some racks were removed and examined in August 1999 and others in February, July and September 2000. Severe pitting and crevice corrosion were noted on most of the aluminium coupons at the CSF, where the water was the most aggressive.
Much of this pitting on the external surfaces was caused by the deposition of what was thought to be iron oxide particles from the corrosion of the carbon steel cover plates.
Some pitting was noted even in the high purity water of the RA6 reactor pool. This pitting was always associated with deposited particles. Pits of 1–2 mm in diameter were produced in waters with no detectable amounts of chloride or sulphate and with a conductivity of less than 2 µS/cm. The particles were thought to be iron oxide and were cathodic to the aluminium. Conditions inside some of the crevices resulted in oxide patches. Other areas seemed pickled, possibly by acidification of the water within the crevices. Small pits (0.1 mm in diameter) were found inside these crevices. The results of these investigations showed that the basin storage environment contained floating particles and it was not always possible to associate the degree of pitting with water composition.
1.3.4.2. Instituto de Pesquisas Energéticas e Nucleares (IPEN), São Paulo, Brazil
Some visible nodular corrosion products were observed on the external fuel plates of some assemblies stored in the basin, despite a history of good water chemistry. Galvanic couples from two possible sources may have accel- erated the corrosion — the first between stainless steel racks used to store the fuel assemblies and the fuel cladding, and the second due to the presence of silver ions in the water that may have plated out on the aluminium surfaces. The IAEA coupon rack was immersed in September 1996. Water conductivity was maintained at <2.0 µS/cm and the pH was maintained in the range 5.5–6.5. The chloride ion concentration was <0.2 ppm during the exposure period. No corrosion spots were observed on the coupons. The first inspection of the racks was conducted after about six months. All three bimetallic/crevice coupons were stuck together, requiring forcible separation. The aluminium coupons were coated with a grey/white deposit typical of aluminium oxide. The pH was measured inside the crevice and it ranged between 4.0 and 4.5 (acidic). The glass ampoules had a brownish tint, indicative of radiation damage. After these observations, the coupons were reassembled and returned to the basin for continued exposure. After an additional year of immersion, the rack was taken
out of the water at a demonstration during the São Paulo RCM. It was disassembled and examined visually. The white oxide was observed in the crevice. In addition, small metallic particles were embedded on the surface of some of the aluminium coupons. The particles were believed to be cathodic, and possibly iron, as a halo existed around the particle/pit. This halo area was shiny and appeared to be free of corrosion.
IAEA racks 2 and 3 were immersed in the basin in August 1998 and were removed in October 1999. Examination of the coupons revealed a few small pits within the crevices of the bimetallic coupons and more pits at the contact points with the ceramic insulators used to separate the coupons. Some loose deposits were seen on the top surfaces of the coupons. The top surfaces of the 1100/1100 couples had such deposits and more than 50 pits of less than 1 mm diameter. The bottom surfaces were free of pits.
An IPEN rack containing 1060, 6061 and 6262 alloys, used in fuel assembly manufacture, was also immersed in the basin. After 16 months of exposure, it was observed that some pitting had occurred on the uncoupled coupons, mostly on the top surfaces. The aluminium couples were stained inside the crevices but were not pitted. The stainless steel–aluminium galvanic coupons were much more severely corroded. Additional laboratory tests were conducted to determine whether increased levels of silver in the basin water could have increased the corrosion of the aluminium cladding in the IPEN basin. Results indicated no pitting but an increase in darkness of the surface oxide colour with the increase of silver concentration.
1.3.4.3. China Institute of Atomic Energy, Beijing, China
Five racks were immersed in the spent fuel storage pool, including the rack provided by the IAEA in 1996. The basin water was not constantly circu- lated and it was purified once a year using an ion exchange system. The conduc- tivity of the basin water ranged from 3 to 10 µS/cm and the chloride ion content was <0.1 ppm. A dark grey oxide layer, from general oxidation, developed on most of the outer surfaces of the coupons in contact with the water. The colour darkened and the layer thickened with time. Crevice corrosion products formed in the crevices between coupons. No pitting was observed on coupon surfaces, but some pitting occurred on the outer rim of the coupons owing to the edge effect.
1.3.4.4. KFKI Atomic Energy Research Institute, Budapest, Hungary
The first inspection and evaluation of the coupons was performed after six months of exposure, in November 1997. The second inspection was carried
out after one year of exposure, in May 1998. Racks 2 and 3 were immersed in the basin in May 1998. Rack 2 was removed in May 1999 and rack 3 in May 2000. The results of the inspections carried out after 6, 12 and 24 months of exposure to the basin water are presented and discussed in Chapter 8. The results obtained from racks 2 and 3 revealed no significant differences between the coupons immersed for one and two years in the pool water.
The results obtained from the evaluation of rack 1 coupons indicate that some corrosion processes were taking place. There were differences in corrosion resistance between the different aluminium alloys. Corrosion of these alloys in the water at the KFKI spent fuel storage pool was probably more dependent on the materials than on the duration of exposure to the pool water.
The duration of one to two years is probably insufficient to show major differ- ences in high quality water.
1.3.4.5. Bhabha Atomic Research Centre, Trombay, India
Rack 1 was immersed in the Mumbai fuel storage basin on 16 January 1997. It included a sample of Al–1S alloy fabricated at the research centre and used as a galvanic couple sample. After eight months, the rack was removed from the water and inspected. Several of the crevice coupons were taken apart, and the pH inside the crevice was measured and found to be 3–4, while the bulk water pH was 5. Some staining was noted inside the crevice. The Al–1S sample coupled to stainless steel showed some white corrosion products on the rim of the sample. Other coupons showed no visible corrosion. The second inspection was made in February 1998 after 13 months of exposure. The rack and coupons were generally free of corrosion products, except for some white product on the rim of the Al–1S specimen and some corrosion inside the crevice. The water conductivity ranged from 2 to 16 µS/cm and the pH from 5.9 to 6.3 during the test. Chloride ion content was always <2.0 ppm. The two additional racks received at the second RCM were immersed in the pool water in July 1998 and removed in August 2000, with an intermediate visual inspection in July 1999.
Individual coupons on these racks had a mirrorlike surface finish. The racks had a total exposure of 742 days.
The coupons were disassembled and photographed during the interme- diate inspection. Unlike the coupons received at the Budapest RCM, which had only a machined surface finish, these highly polished coupons showed no deep pits or crevice corrosion. Basin water conductivity was maintained at 3.5–16 µS/cm, with a chloride ion content of less than 2 ppm. The excellent corrosion resistance of these coupons was attributed primarily to the surface finish and the improved purity of the water.
1.3.4.6. Pakistan Institute of Nuclear Science and Technology, Islamabad, Pakistan
Rack 1 was inserted on 12 November 1996 and was withdrawn for a short time on a monthly basis for visual inspection. No pitting corrosion was seen on any coupons during the 12 inspections. The conductivity of the water ranged from 0.1 to 0.8 µS/cm, with a pH between 4.8 and 6.1. The chloride ion content was <0.5 ppm. Racks 2 and 3, which contained two galvanic couples and three crevice sandwiches, were exposed in October 1998. Rack 2 was withdrawn after one year of exposure, in October 1999. The conductivity of the basin water was always <1 µS/cm, the chloride ion content was <0.05 ppm and the pH was 5.5–6.3.
Coupons were examined every month and discoloration was noted as the coupons developed a general oxidation film with time. No pitting was ever seen on any of the exposed surfaces. Once the coupons were disassembled, some pitting was seen under the ceramic washers of the galvanic coupons. Pitting was also observed under the washers of the crevice coupons. There was no detectable visual change in the stainless steel coupons. The water conductivity was maintained at between 0.1 and 0.7 µS/cm.
1.3.4.7. Research Institute of Atomic Reactors, Dimitrovgrad, Russian Federation
The steel lined storage basin at the research reactor uses ion exchange technology to keep the water purified to a conductivity of 1.4–1.7 µS/cm and a chloride ion content of <20 ppb. Inspections of rack 1 were made after 6 and 14 months. The rack was removed from the pool and dismantled, the coupons were weighed, photographed and reassembled, and the rack was reimmersed in the basin. The coupons freely exposed to water had a uniform grey colour that is typical of general corrosion on some aluminium alloys. Some 0.02 mm deep pits were seen on the exposed surface as well as some small red and brown particles protruding from the coupon surfaces. In addition, a few small, 0.03 mm deep pits were seen at the contact line between the 100 mm disc and the 70 mm disc. There was evidence of crevice corrosion between the coupled coupons.
The weight gain of the alloys as a function of time showed a parabolic behaviour in accordance with the equation Y = ax2+ bx + c.
Racks 2 and 3 were disassembled, and the coupons were examined, weighed and reassembled for continued exposure. Pitting was detected only at the grain boundaries of polished coupons. This was thought to be due to grain boundary etching and not to be caused by the storage pool environment. Some crevice corrosion was noted between the crevice coupons, and some impurities
were seen on the top sides of some of the coupons without pitting. The change in mass of all aluminium coupons was measured and found to be parabolic with time, as expected from the conductivity of the water and the general corrosion or oxidation of the alloys.
1.3.4.8. Russian Research Centre, Kurchatov Institute, Moscow, Russian Federation
During the first 18 months of exposure of the rack, the water conductivity ranged from 1.9 to 7.6 µS/cm and the chloride ion concentration was between
<0.05 and 0.3 ppm. The corrosion rack was immersed in the RR-8 fuel storage basin, and interim inspections were conducted after 6 and 12 months. The basin is lined with stainless steel and has aluminium storage racks. The purification system uses ion exchange filters. A uniform surface oxide with no pitting corrosion was observed during these two inspections. Subsequently, rack 1 was withdrawn at periodic intervals, disassembled and photographed up to a total exposure time of 1254 days. The coupons were weighed, examined, reassembled and immersed again. Two additional racks furnished by the IAEA were immersed in April 1998. The coupons in rack 2 were examined after 367 and 551 days of exposure and those in rack 3 after 725 days of exposure. These coupons were also weighed, photographed and examined for corrosion.
On most of the coupons, general corrosion resulted in a dull grey–white film, with additional corrosion products within the crevices. No pitting corrosion was seen on the outer surfaces of the coupons, except on the outer rim of some of the 6061 crevice coupons and the 6063–AISI 316 galvanic coupons. Corrosion along the rim is quite common, as machined, high energy surfaces are more prone to corrosion. No corrosion was observed on the aluminium specimens inside the glass ampoules. The water and glass were clear, indicating no radiation effects.
1.3.4.9. Office of Atomic Energy for Peace, Bangkok, Thailand
The rack was immersed in the fuel storage pool in November 1996. The coupons were examined visually every 30–40 days for the first six months.
During the first inspection, carried out after 40 days, the crevice coupons were found to be stuck together. A few corrosion nodules were detected after about four months. After six months of exposure, metallographic examination of three coupons was carried out, and the maximum pit depth was found to be 10–40 µm. On the crevice side, the pit density was higher (4–6 pits/cm2) than on the non-crevice side (1 pit/cm2). However, the pits on the non-crevice side were deeper. The 1100 coupons had higher corrosion resistance than the SZAV-1
alloy, and 6061 exhibited the highest corrosion of the three aluminium alloys.
The water conductivity ranged between 1.5 and 4.8 µS/cm, the pH between 6 and 8, and the chloride ion concentration between 0.1 and 0.8 ppm.
Two additional racks of corrosion coupons were immersed in the fuel storage pool in March 1998. The conductivity of the water ranged between 1 and 6 µS/cm during the exposure period and the chloride ion content was between 0.1 and 0.5 ppm. No pitting corrosion was observed on the exposed surfaces of the coupons, except along the outer rim.
1.3.5. General comments on the CRP
(a) The pH of the water and the specimens inside the glass ampoule did not show any change. These specimens were designed for the evaluation of radiation effects.
(b) The colour of the exposed aluminium alloy surface varied from metallic bright to dark grey. The extent to which the surface darkened was dependent on the alloy composition.
(c) Sediments were observed on the top surfaces of many coupons.
(d) A number of participants reported corrosion along the outer rim of the coupons. This would be expected from end grain attack on cut surfaces.
(e) Highly polished coupons were more resistant to corrosion than the as-machined coupons.
(f) The crevice/bimetallic couples were often stuck together with corrosion products and required forcible separation.
(g) The pH in the crevice was generally 0.5–1.0 unit less than in the bulk water.
(h) Pits of less than 0.5 mm diameter were observed on the aluminium in regions in contact with the ceramic separator.
(i) Sediments on the top surfaces of aluminium alloy coupons caused pitting.
No pits were observed on the bottom surfaces of these coupons.
(j) The surface features of coupons exposed for 13 months were similar to those of coupons exposed for 25 months. This timescale had no significant effect on aluminium coupon corrosion.
(k) The crevices of the aluminium alloy couples were stained but not pitted, whereas the aluminium–stainless steel couples were heavily pitted.
1.4. SRS CORROSION SURVEILLANCE PROGRAMME
The corrosion surveillance programme at SRS was established in 1992 to monitor production fuel in on-site spent fuel storage basins [1.7]. This extensive
programme, still in progress, has helped to increase understanding of the corrosion of aluminium clad spent fuels. The data from corrosion surveillance coupons in the SRS basins, the SRTC laboratory tests and detailed reviews of aluminium alloy corrosion from the literature have been documented in a number of publications. Some of the data are included in this report. The data from the SRS surveillance programme have been shared with the IAEA CRP with a view to increasing the understanding of aluminium corrosion and devel- oping a basis for corrosion protection of spent fuels. A description of the SRS programme and the main results are presented below.
1.4.1. Background
In 1989, processing of aluminium clad production fuel was suspended at SRS in order to carry out safety upgrades at reprocessing facilities located in Canyon F and because of issues related to US non-proliferation concerns. The irradiated fuel and target materials were caught in the back end of the nuclear pipeline with no plans for processing. Normal water storage times of 9–18 months became years. Also, with less than optimum water quality during the early 1990s, pitting corrosion of the fuel (Fig. 1.3) became an issue at SRS and at other USDOE sites [1.8]. An extensive programme was initiated at SRS to clean up the storage basins and to install new water purification equipment. At the same time, a corrosion surveillance programme was started to monitor the fuel stored in the basins and to measure the effectiveness of the cleanup activ- ities. The results of these surveillance activities up to 2000 are reported in a number of publications [1.9–1.11].
FIG. 1.3. Nodular corrosion on aluminium clad U–Al alloy fuel.
The SRS corrosion surveillance programme was expanded in 1996 to support the decision of the USA to bring back about 15 000 foreign research reactor fuel assemblies for storage in the Receiving Basin for Off-Site Fuel (RBOF) and the L Reactor basin. In addition, the IAEA extended the scope of its spent fuel management programme to include programmes focused specifi- cally on spent fuel from research and test reactors.
1.4.2. Component immersion tests
The corrosion surveillance programme at SRS, established in 1992, was initiated at a time when corrosion of fuel cladding and aluminium components became evident for the first time. Detailed discussions of the surveillance activ- ities have been presented in Ref. [1.9]. The programme was initially set up to monitor the production fuel and target material from the last irradiation campaigns in the P, K and L Reactors at SRS. The programme was expanded to include the RBOF, in which all the fuel received from off-site locations around the world was stored.
The initial corrosion test racks were made with tube ends cut from un- irradiated fuel and target tubes (Fig. 1.4) They were pretreated to 95°C in deionized water for 30 h to develop a 1 µm thick high temperature boehmite (aluminium oxide) layer on the surfaces.
The racks were initially placed in the K Reactor basin only. There were five withdrawals during 1992, and aggressive pitting corrosion was detected on the surveillance coupons. Pitting corrosion penetrated 0.75 mm, equivalent to a
FIG. 1.4. Component immersion test rack.
fuel cladding thickness, into 8001 alloy in 45 days and into 1100 alloy in about six months. During this time the basin water conductivity was approximately 200 µS/cm and the average chloride content was about 8 ppm. Owing to the aggressiveness of the water towards aluminium components stored in the basin, a new basin management programme was initiated to improve the quality of the environment.
A major effort was initiated to deionize all three reactor basins on a continuous basis using portable deionizing systems, which had been in use since the mid-1960s. This effort resulted in some improvements in all three basins, but because of the limited number and availability of portable deionizers for the three basins, the improvements were slow, and new mixed bed deionizers were acquired for the K and L basins.
The component immersion tests were expanded to include the RBOF and the P and L basins in 1993. In addition, tests were continued in the K basin.
Surveillance coupons were withdrawn periodically from the four basins during 1993–1995, while continuous deionization of the water was being carried out.
With water conductivity lowered to 100–125 µS/cm or less in the L and K basins and the aggressive impurity ion content reduced, no pitting corrosion was seen on corrosion coupons in these basins. The water conductivity in the RBOF has always been maintained in the 1–3 µS/cm range, with the chloride ion content in the ppb range. No pitting corrosion of surveillance coupons has ever been seen in the RBOF under these high quality water conditions. With limited deionizer availability, the water conductivity of the P basin increased to 160 µS/cm, and once again some pitting was observed. A decision was subsequently made to close the P basin and transfer the fuel stored in that basin to the L and K basins.
In the interim period before the new deionization equipment for the L and K basins was received, portable equipment was installed in July 1995 and used to lower the L basin water conductivity from 110 to below 8 µS/cm in 2.5 months. The equipment was then moved to the K basin, and within three months the conductivity was lowered to below 10 µS/cm. Continued deioniza- tion in both basins for two more months lowered the conductivity further, to less than 3 µS/cm, and the chlorides, nitrates and sulphates were lowered to about 0.5 ppm. The corrosion surveillance programme continued in the three reactor basins and in the RBOF while the basin and water quality improve- ments were being carried out, i.e. until mid-1996. Results of the component immersion tests through September 1997 (the last withdrawal) showed no pitting corrosion on any of the corrosion coupons. These coupons were exposed to a variety of conditions for 37–49 months as conditions improved in the basins. Table 1.1 presents a summary of component immersion tests for the period 1992–2000, when corrosion coupons accumulated exposure time in extremely high quality water and withdrawal intervals were extended.
